1. Field of the Invention
The present invention relates to a method for creating a vacuum-tight butt brazing joint between a metallized ceramic part and a part made of stainless steel.
2. Prior Art
Generally a bond between a ceramic material and a metal presents problems, due in particular to the large differences in the coefficient of thermal expansion between the two materials. In order to cope with these problems there have been developed special bonding alloys, mainly iron, nickel and cobalt-based metal alloys, which provide, satisfactory results because the coefficients of thermal expansion of these metal alloys correspond rather well with the coefficient of thermal expansion of ceramic material.
If, however, it is necessary that the bonds display anticorrosive properties, these alloys will not be satisfactory because of their deficient corrosion resistance. Although it has been attempted to overcome this problem by coating the bond with a copper or gold film, i.e., in order to provide a corrosion-resistant film over the bond, such a treatment is rather costly.
German Auslegeschrift No. 2,021,396 discloses a metal-ceramic bond in which the metal comprises more than 50% of nickel and the ceramic material comprises a magnesia. Although the produced bond possesses mutually rather well adapted coefficients of thermal expansion and has moreover a good corrosion resistant character, this metal alloy is, unfortunately, rather high priced. Furthermore, the magnesia possesses a low mechanical strength, a low thermal shock stability and a high reactivity with respect to metals.
A considerably cheaper metal which, especially in view of its corrosion resistant properties, is very well suited for vacuum applications, is austenitic stainless steel, as may be apparent among others from an article in "Vacuum", vol. 26-7 by C. Geyari "Design consideration in the use of stainless steel for vacuum and cryogenic equipment" delivered on the occasion of the third Israeli vacuum meeting at Haifa in September 1973. The use of austenitic stainless steel for a brazed metal-ceramic bond is, however, hampered by the bad brazability of the austenitic stainless steel and the large difference in expansion between the austenitic stainless steel and the ceramic material. However, in view of the benefits of austenitic stainless steel research has been conducted in order to find solutions to allow using this material nevertheless.
With respect to the foregoing, German Auslegeschrift No. 1,045,305 suggests a method for creating a brazing bond between two materials having strongly different coefficients of thermal expansion, as is the case for instance for iron and a ceramic material, wherein an intermediate layer is located between the iron and the ceramic material, the material of this layer having such a coefficient of thermal expansion that the expansion of the metallic part at the location of the joining interface with the ceramic part is compensated for, at least partially.
Assuming a suitable thickness of the intermediate layer, one attains the effect as though at the location of the joining interface there has been fastened a material having a coefficient of thermal expansion substantially the same as that of the ceramic material. However, this known method is rather elaborate.
Another method for creating a brazing bond between a ceramic material and stainless steel has been indicated in a brochure of the firm Friedrichsfeld. In accordance with the latter, the stainless steel is bonded indirectly to the ceramic material through a thermally adapted material, such as for instance NiFe42. This method, however, is unfortunately rather laborious and consequently relatively costly.
The object of the present invention is to provide a method of the above-mentioned type in which the aforesaid drawbacks have been eliminated.